1. Field of the Invention
The present invention relates to a sheet processor and to an image forming apparatus equipped with a sheet processor and, more particularly, to a sheet processor adapted to punch a sheet of paper after an image has been formed thereon.
2. Description of the Related Art
Hitherto, in some image forming apparatuses, such as copying machines, printers, facsimiles, or equipment combining these machines, the main unit thereof is provided with a sheet processor having a punching unit for punching a sheet of paper on which an image has been formed.
As a sheet processor equipped with such a punching unit, there is one disclosed in, for example, Japanese Patent Laid-Open No. 2000-153953. The sheet processor disclosed therein is provided with a punching unit adapted to permit a punching position to be controlled in a sheet conveying direction and in a direction orthogonal to the sheet conveying direction (hereinafter referred to as “the width direction”).
The foregoing sheet processor has as many punches and dies as the number of the punch holes to be made in a sheet. The sheet processor includes a punching unit that moves the punches to the position of the holes in the dies so as to punch a hole in the sheet, an edge-in-the-conveying-direction detector for detecting the leading or trailing end of the sheet in the conveying direction, an edge-in-the-width-direction detector for detecting the side edge of the sheet in the width-wise direction, and a controller for controlling the punching unit in response to signals from the detectors, thus permitting highly accurate punching at preset positions.
With the recent trend toward improved binding quality, becoming available are apparatuses that permit binding in which inserting sheets of paper, such as index paper with tabs or chapter paper, are interleaved in a bundle of sheets of paper, and the inserting sheets of paper and plain paper are stapled or punched together. For the inserting sheets of paper, commercially available paper, customized paper, or the like are used, and in some cases they are pre-punched sheets of paper.
A conventional sheet processor, however, is set to perform the same punching operation for any types of paper. Hence, when unpunched plain paper is punched, inserting paper will be punched also.
As a result, the punched holes made by the sheet processor are added to the pre-punched holes in the inserting paper, causing the final holes to be excessively large. This has been adversely affecting the alignment for filing and contributing to poor appearance, with consequent poor binding quality.